Research Highlights

Nano-encapsulation of Essential Oils: A Unique Approach to Soybean Disease Control

By Laura Temple

Plant essential oils are sometimes used to address many issues, from allergies to mold infestations. They also have potential to help manage soybean diseases. Essential oils distill or concentrate a variety of chemical compounds from plants into a liquid that doesn’t mix with water. 

“Essential oils are known to be antimicrobial and antifungal,” says Leonor Leandro, professor of plant pathology at Iowa State University. “But they are volatile, meaning they are hard to keep in place. That’s why we smell them.”

In much of her research, Leandro focuses on sudden death syndrome, or SDS, a soilborne disease caused by the fungus Fusarium virguliforme. It is one of the most costly soybean diseases in the Midwest. She is exploring creative ways to manage this disease, including determining which plant oils effectively control SDS through a project funded by the Iowa Soybean Association. 

“I learned that essential oils have been used as a post-harvest biopesticide to protect fresh fruit and vegetable quality, and I wanted to learn how they could be used in the field on growing plants,” she says. “To protect soybeans against SDS, an essential oil biofungicide would be applied as a seed treatment. A key challenge would be keeping a volatile oil in place.”

To tackle that issue, Leandro is partnering with Lingling Liu, an adjunct assistant professor of ag and biosystems engineering at Iowa State University. Liu uses plant residue to create nanocellulose, an organic material that can encapsulate bioactive compounds as nanoparticles, measured in billionths of a meter.

“This encapsulation technology has the potential to keep essential oils contained in place,” Leandro explains. 

Testing Effective Essential Oils

Leandro and her team are screening 16 different essential oils with known strong antimicrobial properties for activity against F. virguliforme. They grow the fungus in the lab on culture media amended with different rates or concentrations of the essential oils to determine their effectiveness in reducing fungal growth. 

Some oils and doses show fungistatic activity, preventing the fungus from growing and spreading. Others also kill the fungus, proving to be fungicidal. The next step will be to see how these oils impact soybean germination and early growth.

“Because they are volatile, essential oils have the potential to be phytotoxic to other plants at high doses,” she explains. “Using nanotechnology to deliver treatments means we can use very low doses, in the parts per million range. The oils will be less toxic at those levels.”

Roughly 18 months into the project, her team found lemongrass oil and cinnamon bark oil to have the most activity against the pathogen causing SDS, with clove and thyme oils also showing promise. Rates of just 300 to 500 ppm, or 0.03 to 0.05%, have proven effective. 

“We have started looking at the phytotoxicity of lemongrass and cinnamon bark oils to soybean seed germination in the lab,” Leandro says. “We plan to bring the oils with the strongest activity against SDS and least impact on soybeans forward for greenhouse testing with our unique delivery system.”

Innovative Delivery

Nano-emulsion technology captures compounds like essential oils in droplets just 1 to 100 nanometers across. One nanometer is one billionth of a meter, or less than half the diameter of a strand of DNA. These droplets are so small that they cling to the slightest crevices of a soybean seed and penetrate tissue effectively. At this size, the droplets increase the stability and absorption rates of encapsulated products, while more effectively controlling their release.

“Nanoencapsulation doesn’t work for all compounds, depending on their chemical structure or properties,” Leandro says. “For example, it doesn’t work with fungicide seed treatments that have proven activity on SDS. We hope our research will lead to an antimicrobial treatment that can be combined with a reduced rate of commercial fungicides.”

Liu uses nanocellulose derived from crop residue to create the encapsulating material. For this project, her lab prepared the gel-like nanocellulose from soybean stover and corn stover. That gel shows great promise for holding nanoparticles of effective essential oils in place on soybean seeds. 

Liu and Leandro will test the nano-encapsulation process with the most promising essential oils by coating soybeans, placing them in germination papers and planting them in soil. Their team will monitor seed germination, phytotoxicity and efficacy against F. virguliforme. 

“Once we verify the delivery mechanism works and identify the most effective essential oils and dosage rates, we can work with an interested group to commercialize the concept and make this SDS control option available to farmers,” Leandro says.

This research on SDS is also laying the foundation to apply a similar approach to identifying essential oils effective against other pathogens. For example, the United Soybean Board is funding research for potential solutions to control Pythium, Phytophthora and stem disease-causing pathogens. 

Other Resources:

Nanotechnology Advances Soybean Input Delivery – SRIN article

Sudden Death Syndrome – SRIN Information page

Published: Aug 14, 2023